Prior art arrangements for intracavity third-harmonic generation in CW lasers result in relatively inefficient conversion of fundamental radiation to the third harmonic. By way of example, ultraviolet (UV) radiation having a wavelength of 355 nanometers (nm) can be generated by frequency doubling fundamental 1064 nm (infrared) radiation in a first optically nonlinear crystal to provide second-harmonic radiation having a wavelength of 532 nm (green radiation), then focusing the 532 nm radiation and the fundamental radiation into a second optically nonlinear crystal to generate the 355 nm radiation. The generated UV power can be estimated, for appropriate focusing of the beams and appropriate choice of propagation direction into the crystal, by an equation:P355=αP1064P532  (1)
Where α has dimensions of Watts×10−1 and has dimensions for typical crystals of between about 10−5 and 10−3 and where P355, P1064, and P532 are the powers for the UV, infrared (IR) and green radiations respectively. In a Coherent® Verdi™V10™, intracavity frequency-doubled, diode-pumped, Nd:YVO4 laser, about 350 Watts (W) of IR radiation having a wavelength of about 1064 nm are circulating in a ring-resonator, and about 10 W of green (532 nm) radiation are generated by frequency-doubling the IR radiation in an optically nonlinear crystal of lithium borate (LBO). If a second optically nonlinear crystal of LBO having a length of 20.0 millimeters (mm) were included in the ring-resonator and the IR and green radiation were focused into that crystal, a value of a of 3×10−4 can be achieved and equation (1) predicts that about 1 W of ultraviolet radiation at 355 nm would be generated. This represents a conversion efficiency of pump-power to third harmonic of only about 2.5%. There is a need for an improvement in efficiency for third-harmonic generation in a CW laser.